I've been thinking since Sims 3 (I didn't play 2) how weird it is that the Sims don't even acknowledge their houses, unless it's dark or there's no wallpaper.
As a human being, I desire a little change once in a while. Paint my room purple, rearrange the furniture. Move to a new house, or add a room, move to a new room. So, I want to try making some custom whims such as "repaint the walls", or "get carpet flooring", or even "add new bedroom". Stuff along those lines. They'd be random, and be in relation to whatever room the Sim is standing in at the time (ideally, anyway, but that could be asking a lot).

I don't want to give away all the details of my idea, but that should be enough to tell me if it's do-able. So... Do you think it can be done? If so, how? Where do you suggest I start?

The kind of things you suggest aren't really testable. About the closest thing you can get is the SituationGoalPurchasedObject tuning. Look at the various tuning descriptions under the SituationGoals section if you want to see the types of things you can test for, but nothing approaches being able to detect painting, adding a room or even getting carpeting